1. Field of Invention
This invention relates to a continuous process for making fibrous cellulose acetate which is useful as a filtering medium for filtering liquids.
2. Description of the Prior Art
Copending application Ser. No. 332,640 filed Feb. 15, 1973, which issued as U.S. Pat. No. 3,842,007, Oct. 15, 1974, describes a new form of fibrous cellulose acetate which closely resembles asbestos in its structure and filtering properties. It comprises rod-shaped fibers intermingled with fine fluffy filaments and, like asbestos, it forms a random matrix of fine, fluffy filaments backboned by larger fibrils when it is deposited from an aqueous suspension onto a filter support. The diameters of the fibrils and filaments of this fibrous cellulose acetate range from about 0.5 microns to about 50 microns and the surface area of the dried fibrous mass ranges between about 35 m.sup.2 /g and about 55 m.sup.2 /g .sup.1. FNT .sup.1 As measured in accordance with "Determination of Surface Area Adsorption Measurements by a Continuous Flow Method" by F. M. Nelsen and F. T. Eggertsen, Vol. 30, No. 8, Aug. 1958, Analytical Chemistry, pp. 1387-1390.
Prior to the present invention the above described fibrous cellulose acetate was prepared batchwise by mixing a 2 to 4% by weight solution of cellulose acetate of about 25 to 45 secs. viscosity in an organic solvent of intermediate polarity such as acetone, acetic acid, methyl acetate and 1, 4-dioxane with a cellulose acetate precipitant such as water and mechanically dispersing the resulting precipitate slurry until the surface area of the precipitate, measured as a dried fibrous mass, was in the 35 m.sup.2 /g to 55 m.sup.2 g range.
The present invention is a continuous process for making the above described fibrous cellulose acetate.